Prior art tunable antennas utilizing MEMS switches include tunable dipoles (which suffer from interference from the DC bias lines for the MEMS switches), ordinary slot antennas (which suffer from limited tunability due to the low-impedance current path around the outside of the slot), and patch antennas (which suffer from limited tunability due to the difficulty of detuning an extended resonant structure by a significant amount without significant design difficulty, as well as problems from MEMS switch DC bias line interference). In one aspect, the present invention employs a single open end on a slot and allows the RF MEMS switches arranged near the open end to provide the greatest amount of tunability because the lack of alternative current paths forces the entirety of the antenna current through a closed MEMS switch. Also, the asymmetric design which may be achieved only requires half as many RF MEMS switches as competing geometries, thus lowering the cost and complexity of the antenna.
The present invention provides a simple way to accomplish several functions that are becoming more important as RF devices continue to become more complex. Cellular telephone manufacturers and RF system designers have already identified antenna diversity as an important addition to future wireless systems, in both the cellular handset and the base station. One way to accomplish this is to switch between several separate antennas. Another way, which is addressed in this disclosure, is to use a single antenna, and to reconfigure that antenna into multiple modes, allowing the apparatus to switch among these modes. This can result in a simpler design that takes up less space, an important consideration especially for the handset application.
The present invention provides a simple antenna that can switch among several different modes through the use of RF MEMS switches. It can provide control over the frequency, pattern, and polarization with a minimum number of switches, in a manner that is simple to design and to manufacture. It also has several advantages over existing alternatives, such as eliminating interference from DC bias lines for the MEMS switches, and having tunability over a broad frequency range. The use of this antenna as part of a diversity transceiver can provide several decibels of improved signal/noise ratio. The antenna geometries presented here have the advantage that they accomplishes this task with a minimum complexity and volume and provide control over a wide variety of antenna properties including frequency, polarization, and pattern.
This invention is applicable to the field of tunable antennas in general, as well as two types of antenna diversity (pattern and polarization). The disclosed antenna can be used in several applications, including automotive communication systems and military communication systems. As cars are beginning to require a greater number of services (onstar, gps, PCs, amps, sdars, etc.) the antenna requirements are becoming increasingly stringent. The use of antenna diversity is already recognized by cellular handset designers as an important advantage and a good way of improving the link budget. Cars, with their increased real estate, are a perfect candidate to take advantage of these new techniques to improve reliability and/or bandwidth. Another possible application is software radio, in which the military is making significant investments as the future solution to all communication needs.
The prior art includes:                (1). Ken Takei, “Tunable Slot Antenna”, U.S. Pat. No. 6,028,561, Feb. 22, 2000, assigned to Hitachi. This antenna consists of a folded, U-shaped slot antenna which is built within a cavity. It is fed from a point in the center of the U-shape by an internal microstrip-like structure which causes a field across the center of the slot. It contains a tunable capacitance by means of a varactor diode, which is connected at the feed point. By applying a DC bias to the varactor, one may tune its capacitance, and thus adjust the input impedance of the antenna. This tunes the frequency where the antenna is matched. The antenna disclosed herein is different from this one in that it uses conductive, metal to metal contact MEMS switches to perform the tuning function. Since varactors can be lossy, the low loss of the MEMS devices provides an efficiency advantage. Furthermore, our design allows the antenna to have a variety of configurations for tuning the radiation pattern or the polarization, as will be explained below.        (2). Hiroshi Okabe, Ken Takei, “Tunable Slot Antenna with Capacitively Coupled Island Conductor for Precise Impedance Adjustment”, U.S. Pat. No. 6,034,644, Mar. 7, 2000, assigned to Hitachi. This design is very similar to Takei's original design (See Ref(1)), but he has moved the varactor to within the slot, and added another branch to the slot. The disclosed antenna still has the same advantages as in the previous case.        (3). Hiroshi Okabe, Ken Take, “Tunable Slot Antenna with Capacitively Coupled Island Conductor for Precise Impedance Adjustment”, U.S. Pat. No. 6,188,369, Feb. 13, 2001, assigned to Hitachi. This design is nearly identical to the previous patent by the same authors. Our antenna still has the same advantages as in the previous case.        (4). Hiroshi Okabe, Ken Take, “Wireless Handset”, U.S. Pat. No. 6,198,441, Mar. 6, 2001, assigned to Hitachi. This patent describes a method of tuning a handset antenna to the frequency on which a call is being placed. The tuning function is controlled by the phone circuitry so that it will intelligently synchronize the antenna frequency with the frequency of the present telephone call. However, modern cellular telephones use spread spectrum techniques, so this type of design is not particularly useful. In the present design, one can not only switch bands, but one can also switch the radiation pattern or the polarization.        (5). Robert Snyder, James Lilly, Andrew Humien, ““Tunable Microstrip Patch Antenna and Control System Therefore”, U.S. Pat. No. 5,943,016, Aug. 24, 1999, assigned to Atlantic Aerospace Corporation. This patent describes a method of tuning a patch antenna by using RF switches to connect or disconnect a series of tuning stubs. The presently disclosed antenna provides several advantages over this design. Since the present antenna uses a slot for the fundamental element, it is less sensitive to the position of the bias circuits. Furthermore, the disclosed design adds several features such as the ability to tune the polarization and the pattern, in addition to the frequency.        (6). Trent Jackson, William McKinzie, James Lilly, Andrew Humen, ““Tunable Microstrip Patch Antenna and Control System Therefore”, U.S. Pat. No. 6,061,025, May, 9, 2000, assigned to Atlantic Aerospace Corporation. This patent is basically the same or very similar as the previous patent by some of the same authors.        (7). Jeffrey Herd, Marat Davidovitz, Hans Steyskal, “Reconfigurable Microstrip Array Geometry which Utilizes Microelectromechanical System (MEMS) Switches”, U.S. Pat. No. 6,198,438, Mar. 6, 2001, assigned to The United States of America as represented by the Secretary of the Air Force. This patent describes an array of patch antennas which are connected by RF MEMS switches. The device provides a tunable antenna which is tuned by selectively turning on or off various switches to connect the patches together. Larger or smaller clusters of patches will create antennas operating at lower or higher frequencies, respectively. One problem with this design is that it requires a large number of switches. A more significant problem is that it does not provide a way to eliminate the problem of interference between the DC feed lines and the RF part of the antenna.        (8). Gerard Hayes, Robert Sadler, “Convertible Loop/Inverted F Antennas and Wireless Communicators Incorporating the Same”, U.S. Pat. No. 6,204,819, Mar. 20, 2001, assigned to Telefonaktiebolaget L. M. Ericsson. This patent describes an antenna incorporating MEMS devices which are used to tune the resonance frequency by selectively activating various portions of the antenna. One drawback of this design is that it is complicated to design, as each resonant section is built of a different type of antenna. Furthermore, it only allows for frequency tuning. The present design allows for polarization tuning or pattern tuning in addition to frequency tuning, and it performs these functions using a simple structure that is easy to design.        (9). Frank Schiavone, “Linear Polarized RF Radiating Slot”, U.S. Pat. No. 4,367,475, Jan. 4, 1983, assigned to Ball Corporation. This patent describes a slot antenna having two open ends, in which the frequency of the slot antenna is determined by lumped elements placed within or around the slot. Our antenna improves upon this design by making it tunable through the use of RF MEMS switches. The present antenna provides a structure that is unlike both the closed-ended slots of traditional slot antenna designs, and also unlike this open-ended slot design. The present antenna is closed, but only by the MEMS switch, and thus forces all of the antenna current to pass through the switch.        (10). David Haub, Louis Vannatta, Hugh Smith, “Multi-layered compact slot antenna structure and method”, U.S. Pat. No. 5,966,101, Oct. 12, 1999, assigned to Motorola. The patent in its prior art portion shows the concept of an open-ended slot antenna. The basic concept of the single open-ended slot antenna does not anticipate the ability to tune the antenna through the use of RF MEMS switches. The open-ended slot antenna shown in this prior art reference is typically one-quarter wavelength long. Our MEMS-tuned slot antenna is one-half wavelength long, like a conventional slot antenna, but the use of the open end forces the entirety of the antenna current to pass through the MEMS switch.        
The disclosed antenna provides several important advantages over the prior art. These include (1) the use of RF MEMS switches which allow the antenna to have higher efficiency than other designs that use lossy varactors, (2) the ability to tune not only the frequency, but also the pattern or the polarization, (3) a simpler design than many of the alternatives, which requires very little fine-tuning or experimentation to arrive at the correct geometry, (4) the versatility to be used in a broadband (without cavity) or narrowband (with cavity) applications with little modification to the design. Previous attempts to provide tunable antennas, including MEMS tuned antennas, can be found in the prior art listed above. None of these examples have the simplicity or versatility of the present design. The most obvious MEMS tuned antennas include the MEMS tuned dipole or patch or an ordinary slot antenna. Each of these have drawbacks including interference from DC bias lines of the MEMS switches or a limited tuning range due to the geometry of the antenna. The present antenna, which preferably includes an open-ended geometry, forces all (or at least most) of the antenna current to pass through a closed MEMS switch, resulting in the greatest frequency tuning range. Prior art exists which describes open ended slot antennas; however, the present antenna functions like a conventional slot antenna, because it is really closed at both ends—one end may be closed by the continuous ground plane itself and the other end may be closed by a closed RF MEMS switch. Additionally, the position of the end which is closed by the closed RF MEMS switch is movable, because various MEMS switches can be closed can be disposed at various positions. This differs from previous attempts to achieve the same effect, because all previous attempts at MEMS tuned slot antennas were of the closed-closed design, which resulted in limited tunability. The present design solves this problem by using a closed-open design, where the open end is then actually closed by a MEMS switch and preferably by one of a plurality of MEMS switches. The particular MEMS switch which is closed of the plurality allows the frequency of operation of the slot antenna to be controlled.